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Monday, May 20, 2013

I thought I had a good theory. Sea water is a buffered solution so when you start to add acid (like CO2 which makes carbonic acid when it dissolves in water) the pH should only change very gradually until the first buffering ion, whatever that is, is used up. Then a little more acid should send the pH plummeting until you start to use the next ion in the sequence. In order to test this a litre of filtered sea water was put on a stirring plate with a Teflon covered magnet in the water. A burette was filled with very dilute HCl and used to titrate the sea water. A pH meter was used to measure the acidity of the sea water and after each drop, the pH metre was allowed to stabilize. The pH for each drop was recorded but only every tenth one is shown on the following graph to avoid the graph becoming too busy.

The HCl was not titrated to determine it's strength since at this stage it was only desired to see if the hypothesized steps exist. If found, a second trial would have been done with Hydrochloric acid of a known concentration.

As can be seen from the graph, pH decreases rather rapidly right from the start and levels out a little for a while after which it once more drops more steeply. The thought behind this trial was that with the addition of Carbon dioxide to the oceans, pH should not change much up to a certain point at which time a little more would send pH rapidly downward with disastrous consequences for the calcium carbonate depositing organisms of the oceans. This is apparently not so. pH drops rather rapidly right from the start.

There is a possibility that Carbon dioxide does not react the same way as Hydrochloric acid. Hydrochloric acid is a strong (completely ionized) acid. Carbon dioxide is not. My chemistry is not up to being able to make any suggestions on this.

Incidentally, aragonite dissolves at a pH of around 7.2 to 7.4 depending on the concentration of carbonate in the water. It is not just pH dependent. The graph suggests this may come fairly soon. Estimates I have read suggest that about 30% of the alkalinity of sea water has already been used up.